models: Remove unused ModelReprMixin class.

It appeared to be used as a base class in various Django migrations,
but because it didn't define any model fields, it wasn't actually.

zerver/migrations/0044_reaction.py

@@ -21,7 +21,7 @@ class Migration(migrations.Migration):
                 ('message', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, to='zerver.Message')),
                 ('emoji_name', models.TextField()),
             ],
-            bases=(zerver.lib.str_utils.ModelReprMixin, models.Model),
+            bases=(models.Model,),
         ),
         migrations.AlterUniqueTogether(
             name='reaction',